More attention has been paid to environmental issues in recent years, and a low-pollution water base paint without containing an organic solvent has attracted attention. In addition, with noticeable technical development in water base paints, appearance of high-grade finishing that could conventionally be achieved only by a solvent-type paint can now be realized also by a water base paint.
However, when a metal pigment is used as base particles having a metallic hue, and in particular, when an aluminum flake pigment (aluminum particles) or the like is used as such a metal pigment, the aluminum flake pigment or the like, if it is blended into a water base paint, may react with water in the paint to cause black discoloration or generate hydrogen gas, resulting in lowering of the storage stability of the water base paint.
In order to solve the problem of such lowering of the storage stability (i.e., lowering of the water resistance) that arises when a metal pigment is used in a water base paint, techniques such as a method wherein a metal pigment is treated with a phosphoric acid- or phosphoric acid ester-based additive (Japanese Patent Laying-Open No. 63-054475 (PTL 1), Japanese Patent Laying-Open No. 61-047771 (PTL 2), Japanese Patent Laying-Open No. 07-133440 (PTL 3), etc.); a method wherein a metal pigment is treated with a Mo (molybdenum) compound (Japanese Patent Laying-Open No. 06-057171 (PTL 4)); and a method wherein a metal pigment is coated with an oxide film such as silica, etc. (Japanese Patent Laying-Open No. 2003-041150 (PTL 5), Japanese Patent Laying-Open No. 2004-131542 (PTL 6), Japanese Patent Laying-Open No. 2004-124069 (PTL 7)), and so on have been developed. However, no metallic pigment is yet to be provided that has reached such a level that it satisfies sufficient storage stability of a water base paint, and in addition, sufficient humidity resistance of a coating film.